Blow-molded foam and method of forming the same

ABSTRACT

A blow-molded foam is provided by foaming and blow-molding the resin material including a polyethylene resin. The foamed resin includes the antioxidant by 300 ppm or more in total. The antioxidant is preferably the combination of a phosphorus antioxidant and a phenolic antioxidant. The optimum amount of phosphorus antioxidant to be included is 250 ppm to 3000 ppm, and that of phenolic antioxidant is 250 ppm to 750 ppm. In the manufacture, the collected resin material and the resin including 300 ppm or more of the antioxidant are melted and kneaded and the foaming agent is mixed thereto to provide the foamed resin, and the foamed resin is blow-molded.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is Continuation of a U.S. application Ser. No.15/027,581 filed Apr. 6, 2016, which claims priority to InternationalPatent Application PCT/JP2014/077115 filed Oct. 9, 2014, which claimspriority to Japanese Patent Application Nos. 2013-212527, filed Oct. 10,2013 and 2014-133340, filed Jun. 27, 2014. The entire contents of theseapplications are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a blow-molded foam and a method offorming the same, and particularly to a blow-molded foam includingpolyethylene as a resin material and a method of forming such ablow-molded foam.

BACKGROUND

Known examples of the blow-molded foam include a climate control ductprovided in an instrument panel of a vehicle. The climate control ductoften employs a foamed duct formed by molding the foamed resin material.The foamed duct is light in weight. Such a foamed duct can bemanufactured easily in a manner that, for example, the resin materialsuch as polyolefin resin including a foaming agent is melted and kneadedand then a foamed parison extruded out of a die of an extruder isblow-molded.

The resin material used for the blow-molded foam is often polyolefinresin. In particular, polypropylene resin is generally used. In recentyears, using polyethylene resin alternatively has been examined for thepurpose of reducing the material cost (see Japanese Unexamined PatentPublication No. 2011-194700 A).

Patent Literature 1 has disclosed the duct for vehicles, which isobtained by adding a chemical foaming agent to a mixed resin obtained bymixing high-density polyethylene with a long-chain branched structure, aspecific gravity of 0.95 to 0.96, a melt flow rate (MFR) of 3 to 7 g/10minutes, and a melt tensile force of 100 to 250 mN and high-densitypolyethylene with a melt flow rate (MFR) of 0.3 to 1.0 g/10 minutes, andthen blow-molding the resulting mixture.

SUMMARY OF THE INVENTION

In regard to the blow-molded foam including the polyethylene resin, theoptimization of the resin composition, the physical properties, etc. isinsufficient as compared to the polypropylene resin. In the status quo,the development thereof has been delayed. However, the blow-molded foamincluding the foamed polyethylene has the high potential demand ascompared to the blow-molded foam including the foamed polypropylene.This is why the development thereof has been anticipated.

From the aspect of the development of the blow-molded foam including thepolyethylene resin as the foamed resin, the foamed resin including thepolyethylene resin has a problem that a pin hole (or imperfect moldingdue to a pin hole) easily occurs. This problem is more remarkable inmolding the highly foamed article and the article with the complicatedshape. Another problem is that the production stable over a long periodis impossible. Various examinations have been conducted in theconventional development in regard to the use of supercritical foamingagents, the use of the material with the high melt tensile force, theadjustment of the molding condition, etc. However, the satisfyingsolution has not been found yet.

The present invention has been made in view of such circumstances. Anobjective of the present invention is to provide a lightweight andinexpensive blow-molded foam with the excellent quality without a pinhole or the like and a method of forming the same. Another objective ofthe present invention is to provide a blow-molded foam that can beformed by recycling the used polyethylene resin, and a method of formingthe same.

A blow-molded foam according to the present invention for achieving theabove objective is a blow-molded foam formed by blow-molding a foamedresin including polyethylene. The blow-molded foam is featured in thatthe foamed resin includes an antioxidant, and the antioxidant isincluded by 300 ppm or more in total.

A method of forming a blow-molded foam according to the presentinvention is a method of forming a blow-molded foam which includesblow-molding a foamed resin including polyethylene. A collected resinmaterial is mixed with an unused resin including an antioxidant of whichamount is determined so that the antioxidant is added by 300 ppm or morein a blow-molded foam formed by blow-molding. The obtained mixture ismelted and kneaded and furthermore, a foaming agent is mixed in thismixture; thus, a foamed resin is formed. A method of forming ablow-molded foam according to the present invention is featured in thatthe foamed resin is blow-molded.

A foamed duct according to the present invention is featured in that thefoamed duct is formed of a resin including 300 ppm or more ofantioxidant. The present inventor has focused on the characteristic ofpolyethylene, which is different from polypropylene. The presentinventor has found out that the crosslinking deterioration ofpolyethylene by the thermal history, which has not been regarded as aproblem in the conventional non-blow-molded foam, would cause the pinhole or the like in the blow-molded foam. The present inventor hassolved the problems by adding a larger amount of antioxidant.

According to the present invention, the lightweight and inexpensiveblow-molded foam with the excellent quality without a pin hole or thelike can be provided. Moreover, according to the present invention, theused foamed polyethylene resin can be recycled. This can drasticallyimprove the use efficiency of the polyethylene resin.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view illustrating an example of afoamed duct.

FIG. 2 is a schematic sectional view illustrating how to form the ductthrough the blow-molding.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of a blow-molded foam according to the present inventionwill hereinafter be described with reference to the drawings. A foamedduct is taken as an example in the following description.

A foamed duct 10 as a blow-molded foam is configured to let theconditioned air flow from an air-conditioner unit (not shown) to adesired portion along an inner flow path. The shape of the foamed duct10 is not limited to the shape illustrated in FIG. 1. The shape may bearbitrarily determined in accordance with the intended purpose and theplace to install.

The foamed duct 10 according to the embodiment can be obtained by havinga foamed parison, which is formed by extruding a foamed resin from a dieof an extruder, held in a mold and blow-molding the parison. The ductjust after the blow-molding has opposite ends closed. After theblow-molding, the opposite ends are cut through trimming, therebyforming the open shape.

The foamed duct 10 according to the embodiment includes a hollow foamedresin molded article with a pipe wall formed by a foamed layer. Thestructure including the foamed layer with a closed cell structureenables to form the duct with the small weight and the excellent heatinsulating property. The closed cell structure is the structureincluding a plurality of closed cells, and refers to a structure havinga closed cell content of at least 70% or more. Such a structure hardlyallows the dew condensation even if the cool air flows into the foamedduct 10.

The foamed duct 10 according to the embodiment includes a mixed resin asa base material resin. The mixed resin is obtained by mixing a collectedresin material and an unused resin (virgin resin). The foamed duct 10according to the embodiment can be obtained by adding a foaming agent tothe base material resin and then blow-molding the mixture.

A resin molded article is formed by the blow-molding generally in thefollowing manner: a resin material in a melted state is shaped along themold surface, the resin material cooled and solidified is separated fromthe mold, and the burr around the molded article and the opening are cutaway with a cutter or the like. In the manufacture cycle of the massproduction with the blow-molding, the unnecessary part of the resinmaterial that has been melted and solidified around the completedproduct are pulverized and then collected. The collected resin materialis reused from the viewpoint of resource and cost saving. This collectedresin material is mixed with virgin resin which has never been heated,so that the mixed resin is provided. A foaming agent is added to thismixed resin and then blow molding is performed again.

In such a manufacture cycle of the mass production, in some cases, thecollected resin material constitutes as much as 70 to 90% of the resinmaterial used in the molding. Specifically, for example, after theblow-molding, the collected resin material resulting from theblow-molding is extracted and virgin resin is added to the collectedresin material by approximately 10 to 30% relative to the entire resinmaterial of which amount corresponds to the amount extracted for thefinal foamed and molded article, thereby forming the mixed resin, and inthe case of performing the blow-molding again with this mixed resin, thecollected resin material constitutes 70 to 90%.

As described above, the manufacture cycle includes preparing the mixedresin by adding the virgin resin to the collected resin materialresulted from the blow molding, and blow-molding the mixed resin again.As this manufacture cycle is repeated, the properties of the foamedarticle (foamed duct 10) formed by molding the mixed resin are oftenlower than the properties of the foamed article formed by molding thevirgin resin only.

In particular, in the case of using the polyethylene resin as the resinmaterial for the foamed duct 10, polyethylene is oxidized to deterioratedue to the thermal history repeatedly applied, resulting in theformation of the crosslinked substance. This leads a major problem thata pin hole detect occurs starting from the nucleus including thecrosslinked substance of crosslinked polyethylene in the blow-molding.In particular, the defect occurs more remarkably if forming the foamedduct with an expansion ratio (value obtained by dividing the density ofthe resin by the apparent density including the cells) of 1.5 or more.

In view of this, the foamed duct 10 according to the embodiment employsthe polyethylene resin as the foamed resin material and additionallyincludes a large amount of antioxidant, thereby suppressing the defectcaused by the crosslinking deterioration. Description will hereinafterbe made of the foamed resin material used for the foamed duct 10.

First, the polyethylene resin used for the foamed resin material isdescribed. Polyethylene may be low-density polyethylene (LDPE),high-density polyethylene (HDPE), or linear low-density polyethylene(LLDPE). A copolymer of ethylene and another copolymerizable monomer maybe used. In any case, polyethylene with a long-chain branched structureis preferably included. Using polyethylene with the long-chain branchedstructure improves the foaming property.

Polyethylene with the long-chain branched structure (hereinafterreferred to as long-chain branched polyethylene) is, for example,long-chain branched polyethylene as disclosed in JP-A-2012-136598. Suchlong-chain branched polyethylene has a branched structure only at aterminal of a long polyethylene chain. Because of this structure, thelong-chain branched polyethylene is featured in that the number ofbranched structures is fewer than that of normal polyethylene.

The long-chain branched polyethylene can be manufactured throughethylene polymerization with a catalyst including an organic aluminumcompound and an organic modified clay mineral obtained by modifying claymineral belonging to smectite group hectorite with a particular organiccompound.

The long-chain branched polyethylene may have an arbitrary physicalproperty. For example, the density thereof, which is defined by thevalue of the density measured based on JIS K7676, is preferably in therange of 925 to 970 kg/m³, particularly preferably 930 to 960 kg/m³. Thelong-chain branched polyethylene exhibits two peaks in the molecularamount measurement by GPC.

The ratio (Mw/Mn) of the weight-average molecular weight (Mw) of thelong-chain branched polyethylene to the number-average molecular weight(Mn) thereof is 2.0 to 7.0, preferably 2.5 to 7.0, and more preferably3.0 to 6.0. The number-average molecular weight (Mn) measured by GPC ispreferably 15,000 or more, more preferably in the range of 15,000 to100,000, and particularly preferably in the range of 15,000 to 50,000.

The number of long-chain branches of the long-chain branchedpolyethylene is preferably 0.02 or more per 1000 carbon atoms in themain chain. The number of long-chain branches of the fraction with an Mnof 100,000 or more, which is obtained by the fractionation based on themolecular weight, is 0.15 or more per 1000 carbon atoms in the mainchain. The proportion of the fraction with an Mn of 100,000 or more,which is obtained by the fractionation based on the molecular weight, isdesirably less than 40% of the entire polymer.

In the present invention, an antioxidant is added to the foamed resinmaterial. The amount of antioxidant to be added needs to be set so thatthe polyethylene resin (foamed article) includes the antioxidant by 300ppm or more in total, preferably 500 ppm or more in total. If theantioxidant is included by less than 300 ppm, the effect isinsufficient, which may make it difficult to suppress the crosslinkingdeterioration over a long period of time.

The antioxidant may be any known antioxidant. Any of various kinds ofantioxidants may be used alone or in combination with another. Note thatthe experiments of the present inventors have proved that thecombination of particular antioxidants is effective. Description ishereinafter made of the combination of antioxidants.

The antioxidant includes an antioxidant that operates to capture aradical (first antioxidant), and an antioxidant that decomposes aperoxide (second antioxidant). Examples of the former (firstantioxidant) include a phenolic antioxidant and a hindered aminecompound (HALS). Examples of the latter (second antioxidant) include aphosphorus antioxidant and a sulfur antioxidant. The resin generatesradicals by heat, light, or shear. If the generated radicals are left asthey are, the crosslinking deterioration and oxidation deteriorationoccur and such deterioration results in the lower physical properties.The former (antioxidant that operates to capture a radical (firstantioxidant)) operates to capture the generated radical. This operationprevents the crosslinking deterioration and the oxidation deterioration.On the other hand, the latter (antioxidant that decomposes the peroxide(second antioxidant)) operates to decompose the peroxide (radical)generated in the thermal oxidation into alcohol, thereby stopping thechain reaction to deterioration.

As described thus, both the first antioxidant and the second antioxidantare widely used for preventing the oxidation, though their functions aredifferent. As a result of the examination by the present inventors,however, it has been turned out that these oxidants have some problemsif added to the polyethylene resin.

For example, if only the antioxidant that decomposes the peroxide(second antioxidant) is added, performing the recycle test involving thethermal history increases the melt tensile force MT largely after thethermal history, i.e., the oxidation prevention effect tends to beinsufficient. The melt tensile force MT, which is the parameter observedin the oxidation deterioration, is large. That is to say, the pin holeis formed in the molding because of the foreign substance from theoxidation deterioration.

On the other hand, if only the antioxidant (first oxidant) that operatesto capture a radical is added, performing the recycling test involvingthe thermal history results in the lower melt tensile force MT after thethermal history because of the influence from the antioxidant. It hasbeen turned out that the addition in large amount of 500 ppm or moreleads to a problem of lower melt tensile force MT. The lower melttensile force MT causes the parison to fail to follow if the blow ratioin the blow molding is high. In this case, the pin hole is easilyformed.

These findings have concluded that the combination of the firstantioxidant and the second antioxidant provides the sufficient oxidationprevention effect and that the change amount of the melt tensile forceMT can be minimized by offsetting the decrease in melt tensile force MT,which would occur if only the first antioxidant were added, by theaddition of the second antioxidant.

Thus, in the embodiment of the present invention, it is preferable toemploy the combination of the antioxidant (first antioxidant) thatoperates to capture a radical and the antioxidant (second antioxidant)that decomposes the peroxide. This leads to the sufficient oxidationprevention effect and the suppressed deterioration in physical property.

The first antioxidant may be any of the aforementioned phenolantioxidant and hindered amine compound (HALS). The phenolic antioxidantis particularly preferable. The second antioxidant may be, for example,a phosphorus antioxidant or a sulfur antioxidant. The phosphideantioxidant is preferable. The phenolic antioxidant and the phosphorusantioxidant are excellent in practicability because these are easilyaccessible and stably supplied and have high purity. The phosphorusantioxidant has other features of the excellent resistance againsthydrolysis and vaporization. The phenolic antioxidant is effective inimproving the resistance of various kinds of resins and elastomersagainst heat. Since the phenolic antioxidant has high molecular weight,the phenolic antioxidant has features of low extraction and lowvaporization.

As described thus, using the phosphorus antioxidant and the phenolicantioxidant in combination provides the additional effect. This caneffectively suppress the crosslinking deterioration and the oxidationdeterioration of the polyethylene resin, and moreover suppress thechange in melt tensile force MT and the like in the recycling.

The phosphorus antioxidant and the phenolic antioxidant may be any knownantioxidant. For example, the phosphorus antioxidant includes ahigh-molecular phosphorus antioxidant and a low-molecular-weightphosphorus antioxidant. Any one of them may be used alone, or both maybe mixed to be used.

Specific examples of the high-molecular phosphorus antioxidant includetris(2,4-branched C3-8alkyl-butylphenyl)phosphite (such astris(2,4-di-t-butylphenyl)phosphite), and tetrakis(2,4-di-branchedC3-8alkylphenyl)-4,4′-C2-4alkylene phosphite such astetrakis(2,4-di-t-butylphenyl)-4,4′-biphenylene phosphite. An example ofthe commercial high-molecular phosphorus antioxidant is a product“Irgafos168” of CIBA JAPAN.

Examples of the low-molecular-weight phosphorus antioxidant includetriphenyl phosphite, diphenyl isodecyl phosphite, phenyl diisodecylphosphite, tris(nonylphenyl) phosphite, and phosphine compounds such astri-2,4-dimethylphenyl phosphine, tri-2,4,6-trimethylphenyl phosphine,tri-o-tolyl phosphine, tri-m-tolyl phosphine, tri-p-tolyl phosphine,tri-o-anisyl phosphine, and tri-p-anisyl phosphine.

The phenolic antioxidant includes a high-molecular phenolic antioxidantand a low-molecular-weight phenolic antioxidant. Any one of them may beused alone, or both may be mixed to be used.

An example of the high-molecular phenolic antioxidant is a hinderedphenol compound. Examples of the hindered phenol compound includetris(2-alkyl-4-hydroxy-5-branched C3-8alkylphenyl)butane such as1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane,tris(3,5-di-branched C3-8alkyl-4-hydroxybenzyl)benzene such as1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene,1,3,5-trialkyl-2,4,6-tris(3,5-di-branchedC3-8alkyl-4-hydroxybenzyl)benzene such as1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene,tetrakis[alkylene-3-(3,5-di-branchedC3-8alkyl-4-hydroxyphenyl)propionate]C1-4alkane such astetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane,and pentaerythrityltetrakis[3-(3,5-di-branchedC3-8alkyl-4-hydroxyphenl)propionate] such aspentaerythrityltetrakis[3-(3,5-di-t-butyl-4-hydroxyphenl)propionate]. Anexample of the commercial high-molecular phenolic antioxidant is aproduct “Irganox1010” of CIBA JAPAN.

Examples of the low-molecular-weight phenolic antioxidant include:monophenolic compounds such as dibutylhydroxytoluene (BHT), butylatedhydroxy anisole (BHA), 2,6-di-t-butyl-p-cresol,2,6-di-t-butyl-4-ethylphenol, 2,6-di-t-butylphenol,2,4-dimethyl-6-t-butylphenol, 2-methyl-4,6-di-nonylphenol,butylhydroxyanisole, styrenated phenol, 2,4,6-tri-t-butylphenol, and4,4′-dihydoxydiphenyl; bisphenolic compounds such as2,2′-methylenebis(4-methyl-6-t-butylphenol),2,2′-methylenebis(4-ethyl-6-t-butylphenol),4,4′-butylidenebis(3-methyl-6-t-butylphenol),4,4′-butylidenebis(2,6-di-t-butylphenol),1,1′-bis(4-hydroxyphenyl)cyclohexane, and2,2′-dihydroxy-3,3′-di(α-methylcyclohexyl)-5,5′-dimethyldiphenylmethane;hydroquinone compounds such as 2,5-di-t-butylhydroquinone, hydroquinonemonomethylether, and 2,5-di-(tertiary amyl)hydroquinone; and hinderedphenolic compounds such asn-octadecyl-3-(4′-hydroxy-3′,5′-di-t-butylphenol)propionate. Moreover,the low-molecular-weight phenolic antioxidant includes a metaldeactivator such as a hydrazine compound with a hindered phenolstructure such as{N,N′-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyl]hydrazine}.

The amount of phenolic antioxidant and phosphorus antioxidant to beadded needs to be set so that the total amount of antioxidants to beincluded is 300 ppm or more. In addition, the amount of each antioxidantto be added is optimized. Specifically, the amount of phenolicantioxidant to be included is preferably in the range of 250 ppm to 750ppm. The amount of phosphorus antioxidant to be included is preferablyin the range of 250 ppm to 3000 ppm.

The molded foam of the polyethylene resin including the phenolicantioxidant and phosphorus antioxidant in the above range can have theexcellent recyclability and moldability. In particular, the change inthe physical properties before and after the thermal history (melttensile force MT and melt flow rate MFR) can be suppressed, which iseffective in recycling the polyethylene resin.

Specifically, adding the phenolic antioxidant and phosphorus antioxidantin predetermined amount can suppress the change ratio of the melt flowrate MFR in the range of 5% or less. Specifically, the change ratio ofthe melt flow rate MFR refers to the change ratio after three thermalhistories each performed using the resin material (resin materialincluding polyethylene as raw material) including the antioxidantrelative to the melt flow rate MFR after one thermal history. If themelt flow rate MFR has increased by over +5%, it means that the resin isgetting decomposed. This possibly results in the drawdown or makes theresin fragile. On the contrary, if the melt flow rate has decreased byover −5%, it means that the resin is in the situation that oxidationdeterioration has occurred. This possibly leads to the pin hole defectcaused by the foreign substance from the oxidation deterioration.

Similarly, adding the phenolic antioxidant and phosphorus antioxidant inpredetermined amount can suppress the change ratio of the melt tensileforce MT within a range of −5% to 0%. The change ratio of the melttensile force MT refers to the change ratio after three thermalhistories each performed using the resin material relative to the melttensile force MT after one thermal history. If the melt tensile force MThas increased by over 0%, it means that the resin is in the situationthat oxidation deterioration has occurred. This possibly leads to thepin hole defect caused by the foreign substance from the oxidationdeterioration. On the contrary, if the melt tensile force MT hasdecreased by over −5%, the molten resin fails to follow the expansion inthe blow molding. This possibly fails to form a thin portion or causesthe drawdown.

In regard to the optimum value (intermediate value) of the melt tensileforce MT of this material mixture, the value of the resin materialincluding the antioxidants after one thermal history and the valuethereof after three thermal histories are both 180 mN. The resinmaterial with a value apart from 180 mN by ±10 mN or more is determinedas having an adverse influence on the foam-molding cyclability. Forexample, if the melt tensile force MT is more than 190 mN (180 mN+10mN), the molding temperature needs to be set high. The high moldingtemperature may make it difficult to form a complicated shape. On thecontrary, if the melt tensile force MT is less than 170 mN (180 mN−10mN), the drawdown is increased. This may fail the molding with largethickness. The thin portion may have a pin hole.

In regard to the thermal history, the virgin resin including theantioxidants in a melted state is extruded from a predetermined extruderunder a condition shown below, and then solidified; thus, the samplewith one thermal history is obtained. The specific extrusion conditionis as follows: an extruder with a screw inserted, the screw having adiameter of 25 mm and an L/D of 20, is used; the number of rotation ofthe screw is set to 60 rpm; the extrusion port has a slit-like shape of25 mm×1 mm; and the temperature inside the extruder is adjusted toapproximately 200 to 210° C. so that the resin is extruded byapproximately 3 kg/h. The sheet-shaped extruded resin is cooled to besolidified between metal plates. After the sample with one thermalhistory is obtained, all the solidified resin material is pulverizedwithout adding the virgin resin; thus, the collected resin material isprepared. Only this collected resin material in the melted state isextruded from the aforementioned extruder under the same condition andsolidified through the same procedure; thus, the sample with two thermalhistories is obtained. After the sample with the two thermal historiesis obtained, the entire solidified resin material is pulverized withoutadding the virgin resin; thus, the collected resin material is prepared.Only this collected resin material in the melted state is extruded fromthe aforementioned extruder under the same condition and solidifiedthrough the same procedure; thus, the sample with three thermalhistories is obtained.

The amount of antioxidant described above corresponds to the amount ofantioxidant included in the blow-molded foam as the final product (here,the foamed duct 10). The amount of antioxidant included in the moldedfoam is, for example, determined by the quantitative analysis based on,for example, a liquid chromatography. The antioxidant is a purechemical, so that there is a particular holding time (varying dependingon the kind of developing medium and column, which is determined basedon the standard substance). The area and the height of the peak are inproportion to the concentration. Thus, the concentration can be known ifthe calibration curve is formed with the standard sample in advance. Inthe case of the phenolic antioxidant and the phosphorus antioxidant, themolded foam as the final product includes the antioxidants in the sameamount as the amount thereof added in the manufacture.

In the manufacture of the molded foam, the polyethylene resin with thephosphorus antioxidant and the phenolic antioxidant added isblow-molded. In the blow-molding, the foaming agent is used to foam thepolyethylene and the foamed polyethylene is molded. The foaming agentmay be an inorganic foaming agent such as air, carbonic acid gas,nitrogen gas, or water, or an organic foaming agent such as butane,pentane, hexane, dichloromethane, or dichloroethane. Among these, thefoaming agent is preferably air, carbonic acid gas, or nitrogen gas. Byusing any of these, the mixing of organic matters can be prevented andthe decrease in durability can be suppressed.

A method of foaming preferably employs a supercritical fluid. That is tosay, the carbonic acid gas or nitrogen gas in a supercritical state ispreferably used to foam the polyethylene resin. Using the supercriticalfluid assures the uniform foaming. If the supercritical fluid isnitrogen gas, the condition may be as follows: the critical temperatureis −149.1° C. and the critical pressure is 3.4 MPa. If the supercriticalfluid is carbonic acid gas, the condition may be set as follows: thecritical temperature is 31° C. and the critical pressure is 7.4 MPa.

The thusly foamed polyethylene resin is blow-molded by a known method,thereby forming the foamed duct 10. FIG. 2 is a diagram illustrating howto form the foamed duct 10 by blow-molding.

In the blow-molding, first, the resin material (polyethylene resin) tobe used in the molding is kneaded in the extruder to manufacture thebase material resin. If only the virgin resin (unused resin) is used inthe molding, the virgin resin of the polyethylene resin including amodifier added as necessary is kneaded to manufacture the base materialresin. In the case of using the collected resin material, the pulverizedcollected resin material including the virgin resin added in apredetermined proportion is kneaded to manufacture the base materialresin.

In the former case (molding with the virgin resin only), the antioxidantis added to the virgin resin as the raw material and the amount of theantioxidant is determined so that the total amount of antioxidant in theblow-molded foam is the aforementioned amount. In the latter case (usingthe collected resin material), the antioxidant is added in advance tothe virgin resin, and the amount of the antioxidant is theaforementioned amount (for example, 300 ppm or more) in the blow-moldedfoam. The virgin resin including the antioxidant and the collected resinmaterial are mixed and melted and kneaded. Further, the foaming agent ismixed to prepare the foamed resin. The foamed resin is blow-molded.

The base material resin with the foaming agent added thereto is mixed inthe extruder and then accumulated in an in-die accumulator (not shown).Subsequently, after a predetermined amount of resin is accumulated, theresin is pressed down in a direction orthogonal to the horizontaldirection (vertical direction) by a ring-shaped piston (not shown).Then, the base material resin is extruded as a cylindrical parison Pbetween split mold blocks 31 and 32 included in a clamping machine 30from a die slit of an annular die 21 illustrated in FIG. 2 at anextrusion speed of 700 kg/h or more. After that, the split mold blocks31 and 32 are clamped with the parison P between the blocks 31 and 32.In addition, air is blown into the parison P at a pressure ranging from0.05 to 0.15 MPa. Thus, the foamed duct 10 is formed.

The molded resin material is cooled and solidified and a part thereofother than the completed product is pulverized. The pulverized materialis collected to provide the collected resin material. The virgin resinis added to the collected resin material in a predetermined proportion.With the obtained mixed resin, the blow molding is similarly performedagain. Repeating such a manufacture cycle can produce a large amount offoamed ducts 10.

The method of molding the foamed duct 10 is not limited to theaforementioned blow-molding. The vacuum molding that vacuums theextruded parison onto the mold to form the article with a predeterminedshape may be employed alternatively. Another molding is the compressionmolding requiring neither the air blowing nor vacuuming but having theextruded parison held between the mold blocks.

As described above, the foamed duct according to the present inventionhas fewer defects from the crosslinking deterioration, which hasconventionally been a problem unique to the polyethylene resin. This canprovide the foamed duct with the excellent quality and reliability. Inthe case of using the used polyethylene resin, the deterioration problemcan also be solved. Thus, the recycling system can be established.Moreover, the virgin resin can be selected from less expensive materials(polyethylene), so that the further cost reduction is possible.

The present invention has been described so far based on the embodiment.

However, the present invention is not limited to the embodiment above,and various changes can be made within the scope not departing from thecontent of the present invention.

EXAMPLES

Specific examples of the present invention will be described based onthe experiment results.

Experiment 1 Example 1

High-density polyethylene (HDPE) for 40 parts by weight and long-chainbranched polyethylene (product name: 08S55A, TOSOH CORPORATION) for 60parts by weight were mixed. To this mixture, a recycled material(collected resin material) was mixed and kneaded to provide a basematerial resin. With the base material resin, a foamed duct with anexpansion ratio of 2.0 was formed through blow-molding. In the basematerial resin, 240 ppm of phosphorus antioxidant (product name:Irgafos168, CIBA JAPAN) and 160 ppm of phenolic antioxidant (productname: Irganox1010, CIBA JAPAN) were added. The proportion of therecycled material (collected resin material) relative to the basematerial resin was set to 85% in weight ratio.

Example 2

A foamed duct was molded through blow-molding in the same procedure asExample 1 except that the phosphorus antioxidant was added by 600 ppm.

Example 3

A foamed duct was molded through blow-molding in the same procedure asExample 1 except that the phosphorus antioxidant was added by 750 ppm.

Comparative Example 1

A foamed duct was molded through the same procedure as Example 1 exceptthat the antioxidant was not added.

(Evaluations)

The imperfect molding due to the pin hole was not observed in Examples 2and 3. In Example 1, the imperfect molding due to the pin hole was notobserved even after the continuous molding for one week but the pin holedefect was a little seen after one week.

On the other hand, in the comparative example where the antioxidant wasnot added, the imperfect molding was observed in one cycle. The abovefacts indicate that it is effective to add the antioxidant by 300 ppm ormore. The preferable amount of antioxidant to prevent the defectivemolding for a long time is 500 ppm or more.

Experiment 2

Experiment 2 was intended to examine the optimum amount of phenolicantioxidant and phosphorus antioxidant to be added.

The polyethylene resin used in the experiment was obtained by blendinglow-density polyethylene LDPE (product name: G201, Sumitomo ChemicalCo., Ltd.) and high-density polyethylene HDPE (product name: B971, AsahiKasei Corporation) by 50/50. To this polyethylene resin, theantioxidants were added in amount as shown in Table 1. The foamed ductwas blow-molded through the same procedure as Experiment 1.

The amount of included antioxidant is measured as below. First, 100 g ofsample was cut out of the foamed duct. Out of the chopped sample, theantioxidant was extracted using the organic solvent. The extractedantioxidant was developed by liquid chromatography (LC) using theselected appropriate developing medium and column. The holding time,peak area and peak height were measured. The standard substance for eachantioxidant was obtained. The calibration curve was formed using thestandard substance and by applying this curve to the measurementresults, the concentration of the antioxidant was obtained.organic solvent:cyclohexane.2-propanol mixed solution(1:1)  Liquidchromatography measurement condition:developing medium used in stationary placement at 40° C. for 24 hours:acetonitrile detector: HITACHI L-4200 column, Mightysil RP-18PA 4.6mm×150 mm, 5 column temperature: 50° C.

The melt tensile force MT and the melt flow rate MFR of the foamed ductafter the thermal history were measured. From the measurement values,the change ratio was calculated and the results are also shown inTable 1. Table 1 shows the thermal history 1, which corresponds to themeasurement value of the polyethylene resin (virgin resin) melted andextruded once. The thermal history 3 corresponds to the measurementvalue of the polyethylene resin subjected to two more melting andextruding after the thermal history 1. The melt flow rate MFR wasmeasured under the condition of 190° C., 2.16 kg, and g/10 min.

TABLE 1 Antioxidant Melt tensile force MT (mN) MFR (190° C. 2.16 kg g/10min) Phosphorus Phenolic Thermal Thermal Change Thermal Thermal ChangeNo. (ppm) (ppm) history 1 history 3 ratio (%) history 1 history 3 ratio(%)  (1) 0 0 285.9 301.2 5.4 0.33 0.23 −30.3  (2) 100 0 210.7 259 22.90.34 0.27 −20.6  (3) 250 0 197.3 260.8 32.2 0.37 0.29 −21.6  (4) 500 0210.9 249.4 18.3 0.35 0.3 −14.3  (5) 750 0 214.5 271.7 26.7 0.38 0.31−18.4  (6) 1000 0 201 220.3 9.6 0.37 0.35 −5.4  (7) 3000 0 159.5 158.2−0.8 0.333 0.335 0.6  (8) 5000 0 130.4 126.6 −2.9 0.3 0.308 2.7  (9) 0100 188.5 178.8 −5.1 0.4 0.379 −5.3 (10) 0 250 187.8 177.2 −5.6 0.3710.366 −1.3 (11) 0 500 185.8 153.6 −17.3 0.36 0.353 −1.9 (12) 0 750 178.3155.4 −12.8 0.331 0.33 −0.3 (13) 0 1000 178.9 168.6 −5.8 0.293 0.257−12.3 (14) 0 3000 174.5 161.7 −7.3 0.293 0.257 −12.3 (15) 0 5000 149.6148.6 −0.7 0.206 0.194 −5.8 (16) 100 100 183.5 187.7 2.3 0.402 0.386−4.0 (17) 100 250 181.6 186.4 2.6 0.377 0.384 1.9 (18) 100 500 184.7163.8 −11.3 0.379 0.36 −5.0 (19) 100 750 177.9 165.4 −7.0 0.368 0.35−4.9 (20) 100 1000 177.3 161.4 −9.0 0.359 0.344 −4.2 (21) 250 100 186.4187.6 0.6 0.378 0.399 5.6 (22) 250 250 189 182.2 −3.6 0.372 0.363 −2.4(23) 250 500 183.9 176.7 −3.9 0.363 0.37 1.9 (24) 250 750 181.7 174.6−3.9 0.37 0.367 −0.8 (25) 250 1000 175.8 168.9 −3.9 0.365 0.355 −2.7(26) 500 100 182.9 183.3 0.2 0.38 0.38 0.0 (27) 500 250 189.7 182 −4.10.36 0.35 −2.8 (28) 500 500 182.3 178.9 −1.9 0.354 0.34 −4.0 (29) 500750 178.1 177.4 −0.4 0.36 0.359 −0.3 (30) 500 1000 175.6 169.4 −3.50.358 0.355 −0.8 (31) 750 100 188.2 189.2 0.5 0.34 0.34 0.0 (32) 750 250188.2 179.5 −4.6 0.33 0.34 3.0 (33) 750 500 184.3 175.9 −4.6 0.331 0.33−0.3 (34) 750 750 178.8 173.8 −2.8 0.335 0.336 0.3 (35) 750 1000 178.3167.8 −5.9 0.329 0.33 0.3 (36) 1000 250 184.5 185.3 0.4 0.36 0.363 0.8(37) 1000 500 186.7 177.9 −4.7 0.335 0.34 1.5 (38) 1000 750 182.7 176.7−3.3 0.332 0.335 0.9 (39) 1000 1000 177.3 169.2 −4.6 0.332 0.328 −1.2(40) 3000 500 176.4 173.2 −1.8 0.316 0.32 1.3 (41) 3000 750 172.1 170.2−1.1 0.32 0.322 0.6 (42) 5000 500 127.5 125.2 −1.8 0.291 0.292 0.3

As is clear from Table 1, all the requirements: the change ratio of meltflow rate MFR in the range of ±5% or less, the change ratio of melttensile force MT from −5% to 0%, and the melt tensile force MT of 180mN±10 mN are achieved when the phosphorus antioxidant is 250 ppm to 3000ppm and the phenolic antioxidant is 250 ppm to 750 ppm, i.e., No. 22,23, 24, 27, 28, 29, 32, 33, 34, 36, 37, 38, 40, and 41.

On the other hand, for example, in No. 2 to 7 where only the phosphorusantioxidant is added, the increase in melt tensile force MT and thedecrease in melt flow rate MFR in the thermal history 3 are large. Inthese cases, the above requirements are not satisfied. Similarly, in No.8 to 13 where only the phenolic antioxidant is added, the decrease inmelt tensile force MT in the thermal history 3 is observed.

The invention claimed is:
 1. A blow-molded foam formed by foaming andblow-molding a foamed resin with use of a foaming agent made of air,carbonic acid gas or nitrogen gas, wherein the foamed resin comprises apolyethylene resin having a long-chain branched structure and anantioxidant, wherein as the antioxidant, a phenolic antioxidant isincluded in an amount of 250 to 750 ppm relative to the polyethyleneresin, and a phosphorus antioxidant is included in an amount of 250 to3,000 ppm relative to the polyethylene resin; wherein a change ratio ofa melt tensile force MT of the foamed resin after three thermalhistories relative to a melt tensile force MT thereof after one thermalhistory is −5% to 0%, a change ratio of a melt flowrate of the foamedresin after three thermal histories relative to a melt flowrate thereofafter one thermal history is 5% or less; the blow-molded foam is aclimate control duct for vehicles; the foamed resin includes a collectedresin material, and the collected resin material is included by 70% ormore relative to the weight of the foamed resin; and the foaming agentis used in a supercritical condition.
 2. The blow-molded foam accordingto claim 1, wherein a value of the melt tensile force MT of the resinmaterial including the antioxidant after one thermal history and thevalue thereof after three thermal histories are both 180 mN±10 mN. 3.The blow-molded foam according to claim 1, wherein the polyethyleneresin is the only polymer resin in the foamed resin.
 4. A method offorming a blow-molded foam by foaming and blow-molding a foamed resincomprising a polyethylene resin and an antioxidant with use of a foamingagent made of air, carbonic acid gas or nitrogen gas, the methodcomprising: adding, as the antioxidant, a phenolic antioxidant, in anamount of 250 to 750 ppm relative to the polyethylene resin, and aphosphorus antioxidant, in an amount of 250 to 3,000 ppm relative to thepolyethylene resin, in advance to an unused resin, mixing the unusedresin with a recycled material and melting and kneading the mixture,mixing a foaming agent to the mixture to prepare a foamed resin,blow-molding, cooling, and solidifying the foamed resin to obtain amolded article, removing unnecessary parts from the molded article toobtain a completed product, and collecting and pulverizing theunnecessary parts to obtain a new recycled material.
 5. The method offorming a blow-molded foam according to claim 4, wherein the foamingagent is used in a supercritical condition.
 6. The method of forming ablow-molded foam according to claim 4, wherein the polyethylene resin isthe only polymer resin in the foamed resin.
 7. The method of forming ablow-molded foam according to claim 4, wherein a change ratio of a melttensile force MT of the foamed resin after three thermal historiesrelative to a melt tensile force MT thereof after one thermal history is−5% to 0%, and a change ratio of a melt flowrate of the foamed resinafter three thermal histories relative to a melt flowrate thereof afterone thermal history is 5% or less.